Preparation of beta quinacridone pigments

ABSTRACT

A novel method for the preparation of a beta quinacridone pigment is described by the oxidation of 6,13-dihydroquinacridone in the presence of polyvinyl pyrrolidone, a catalyst and optionally a particulate quinacridone or particle growth inhibitor using hydrogen peroxide as the oxidant.

FIELD OF THE INVENTION

[0001] The present invention relates to a novel method for thepreparation of a beta quinacridone pigment by the oxidation of6,13-dihydroquinacridone in the presence of polyvinyl pyrrolidone usinghydrogen peroxide as the oxidant.

BACKGROUND OF THE INVENTION

[0002] The polymorphism of quinacridone pigments is well known. Forexample quinacridone, also referred to as5,12-dihydroquino-[2,3-b]-acridine-7,14-dione of the formula

[0003] (I) is known to exist in three major polymorph modifications. Thealpha (U.S. Pat. No. 2,844,484) and gamma (U.S. Pat. No. 2,844,581, U.S.Pat. No. 2,969,366, U.S. Pat. No. 3,074,950 and U.S. Pat. No. 5,223,624)polymorphs are bluish or yellowish red pigments. The beta polymorph isdescribed in U.S. Pat. No. 2,844,485 as a violet form and in U.S. Pat.No. 4,857,646 as a magenta form while the delta polymorph obtained bysublimation of quinacridone is described as a yellowish red quinacridonein U.S. Pat. No. 3,272,821. Since the alpha and delta forms areunstable, only the red gamma, and the violet and magenta betaquinacridone are of commercial interest.

[0004] Several routes are described for the preparation of quinacridone.Numerous publications describe the oxidation of 6,13-dihydroquinacridoneto quinacridone. For example, U.S. Pat. No. 5,840,901 describes theoxidation of 6,13-dihydroquinacridone using hydrogen peroxide as theoxidant. This unique process provides an economic and environmentallyfriendly method for the preparation of quinacridone pigments.

[0005] Additionally, the process offers the preparation of quinacridonesand its solid solutions in its desired crystal modification as forexample the beta or the gamma quinacridone. For example, U.S. Pat. No.6,013,127 describes a process for the preparation of aquinacridone/6,13-dihydroquinacridone solid solution in its gammacrystal form. Such products show outstanding pigment properties.Generally, a pigmentary form is obtained in the synthesis step withoutthe need of an additional finishing step.

[0006] U.S. Pat. No. 6,225,472 and U.S. Pat. No. 6,406,533 describenovel 6,13-dihydroquinacridone derivatives, which can direct and controlthe growth and/or crystal phase of pigment particles.

[0007] U.S. Pat. No. 6,264,733 describes new pigment particle growthand/or crystal phase directors of the formula(MO₃S)_(m)-Q-(CH₂—(X)—(Y)_(n)o) (II), wherein Q represents a pigmentmoiety, M represents a metal cation, quaternary N cation or H, X is anaromatic group or an aliphatic heterocyclic group with at least one 5atom or 6 atom ring or a heteroaromatic group with at least one 6 atomring and which is not a phthalimido group, Y is a sulfonic acid orcarboxylic acid or salt thereof; m and n independent from each otherrepresent a number from zero to 2.5; and o is a number from 0.05 to 4.

[0008] Although beta quinacridone can be advantageously produced bythese known methods the pigments prepared by such processes are opaqueor semi opaque and still need an after treatment or finishing likesolvent treatment, further grinding or kneading or even precipitationfrom mineral acids like concentrated sulfuric acid to get a very smallparticle size C.I. Pigment Violet 19 in its desired colorcharacteristics, high color strength and high transparency.

[0009] Furthermore, depending on the reaction conditions the betaquinacridone is often obtained in mixture with a small quantity of thered gamma or alpha quinacridone present, leading to a hue shift andlower chroma.

[0010] The present process describes a new route for the preparation ofa pure single phase direct pigmentary beta quinacridone pigment by theoxidation of 6,13-dihydroquinacridone, in that the process is carriedout in the presence of a specific polymer or a specified mixture ofpolymers and optionally in the presence of a small particulatequinacridone as a seed.

[0011] Thus, the inventive process is valuable since it offers a newviable environmentally friendly and economical route for the preparationof new violet quinacridone pigments.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0012] It has now surprisingly been found that quinacridone in improvedbeta pigmentary form can be obtained by the oxidation of6,13-dihydroquinacridone in an aqueous basic methanol media usinghydrogen peroxide as the oxidant and an anthraquinone mono or disulfonicacid as the catalyst in the presence of polyvinyl pyrrolidone andoptionally a small particulate quinacridone as a seed.

[0013] Thus, the inventive process is valuable since it allows themanufacturer to produce high performance, high chroma beta quinacridonepigments in an economical and environmentally friendly manner withoutfurther finishing or solvent treatment. The commercial significance ishigh.

[0014] The present invention relates to a process for preparing aquinacridone pigment of the formula

[0015] (I) in its beta crystal phase, wherein a salt of6,13-dihydroquinacridone of the formula

[0016] (III) is oxidised with hydrogen peroxide in the presence of acatalyst and 0.2 to 4% by weight, preferably 0.5 to 3% by weight, basedon the 6,13-dihydroquinacridone, of polyvinyl pyrrolidone.

[0017] Preferably the polyvinyl pyrrolidone has a molecular weight inthe range of 9,000 to 350,000, most preferably 40,000 to 50,000, and itis added before, during or after the 6,13-dihydroquinacridone saltformation, most preferably before the salt formation.

[0018] Generally, the salt of the 6,13-dihydroquinacridone of formula(III) is a mono- or preferably di-alkali metal salt, or a mixturethereof. Most preferred are the disodium and/or dipotassium salts.

[0019] For example, the 6,13-dihydroquinacridone salt is prepared bystirring the 6,13-dihydroquinacridone in a basic medium, for example amixture of water, alcohol and a base, typically in the presence of thepolyvinyl pyrrolidone, at a temperature of 30° C. or above, preferablyfrom 40 to 100° C., and most preferably from 50° C. and thecorresponding reflux temperature, for 5 minutes to 2½ hours, preferably20 minutes to 1½ hours.

[0020] Furthermore, the presence of small particulate quinacridoneduring the 6,13-dihydroquinacridone salt formation and followingoxidation reaction can have additional beneficial effects for thegeneration of the inventive direct pigmentary violet beta quinacridonepigment.

[0021] Preferably, such small particulate quinacridone has a particlesize of less than 0.2 μm and is in the beta crystal phase as obtainedfor example by known processes from sulfuric acid precipitation or it isa nanosize quinacridone in mixture with other polymers, like naphthalenesulfonic acid formaldehyde polymers as described in example 2 and morein detail in a concurrently filed provisional patent application60/430,522.

[0022] Such small particulate quinacridone is added at a concentrationof 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on6,13-dihydroquinacridone.

[0023] Without limiting this invention to any particular theory, it isbelieved that the nanosize or small particle size beta quinacridonepigment particles can act as seeds for the pigments to be synthesizedand the polyvinyl pyrrolidone can adhere to the synthesized pigmentmolecule and by doing so can further direct the crystal growth andcrystal phase. The term “directing the crystal growth” refers tocontrolling the synthesis of pigment particles to have a suitablepigmentary size as well as directing the growth of the crystals togenerate particles of a specifically desired shape, such as platelet,needle, cubic, leaflet, prismatic and other geometric forms, in adesired crystal phase.

[0024] Generally, the oxidation is carried out in a reaction mediumobtained by combining a slurry, which consists essentially of the6,13-dihydroquinacridone, polyvinyl pyrrolidone, optionally a smallparticulate quinacridone, the catalyst, a base and a suitable liquidphase, with an aqueous solution of hydrogen peroxide.

[0025] In general, a suitable liquid phase is any liquid media whichpromotes the oxidation reaction, and which does not react to asignificant extent with the hydrogen peroxide oxidizing agent. Commonly,the liquid phase is a mixture of a lower alcohol and water whichcontains 20 to 750 parts, preferably 40 to 600 parts of water, and 50 to750 parts, preferably 100 to 600 parts, of alcohol per 100 parts6,13-dihydroquinacridone; all parts being parts by weight.

[0026] The alcohol is generally a lower alcohol, for example aC₁-C₃alkanol, preferably methanol. The reaction medium is preferablysubstantially free of other organic solvents. However, organic solventsare tolerated in the reaction medium as long as they do not impair the6,13-dihydroquinacridone salt generation or the oxidation reaction.

[0027] Any base capable of forming the salt of the6,13-dihydroquinacridone is useful in the reaction medium. Preferably,the base is an alkali metal hydroxide, most preferably sodium orpotassium hydroxide. In certain instances, it is advantageous to use amixture of sodium hydroxide and potassium hydroxide.

[0028] The molar ratio of the base to 6,13-dihydroquinacridone istypically from 1 to 8 moles of base per mole of the6,13-dihydroquinacridone, preferably from 2.2 to 7 moles of base permole of 6,13-dihydroquinacridone.

[0029] The generation of the 6,13-dihydroquinacridone salt is observableunder the light microscope by the formation of crystals of the6,13-dihydroquinacridone salt. Depending on the reaction conditions andthe kind of base, the salt is generally in the form of needles, prisms,cubes or platelets.

[0030] Additionally, it is advantageous to add a particle growthinhibitor before or after the 6,13-dihydroquinacridone salt generationto control the pigment particle size of the oxidized quinacridonepigment. Particle growth inhibitors, also known as anti flocculating orrheology improving agents, are well known. Suitable particle growthinhibitors include, for example, phthalimido methyl quinacridone,imidazolyl methyl quinacridone, pyrazolyl methyl quinacridone,quinacridone sulfonic acid and its salts, for example the aluminiumsalt.

[0031] For achieving an optimum effect, the particle growth inhibitor isadded in an amount of 0.05 to 8% by weight, preferably 0.1 to 5% byweight, based on 6,13-dihydroquinacridone, preferably prior to oxidationafter or before the 6,13-dihydroquinacridone salt generation.

[0032] For various reasons and particularly to avoid potential sidereactions as well as for a more controllable process, the oxidationreaction is preferably carried out under an inert gas flow, for examplean argon or nitrogen flow.

[0033] In an optimized process, the oxidation is carried out bycombining an aqueous solution of the hydrogen peroxide oxidant with aslurry of the 6,13-dihydroquinacridone in a basic mixture of aqueousalcohol, polyvinyl pyrrolidone, base and optionally with a particulatequinacridone and a particle growth inhibitor over a time interval offrom 5 minutes to 6 hours, preferably over 30 minutes to 3½ hours, andsubsequently maintaining the reaction medium at an elevated temperaturewith stirring for a period of time to complete the oxidation and promotepigment recrystallization. Preferably, the reaction medium is maintainedat a temperature of 50° C. or above, most preferably at refluxtemperature, for 5 minutes to 5 hours, preferably 10 minutes to 4 hours,after the addition of hydrogen peroxide. The pigment is then isolated byfiltration, washing with alcohol followed by hot water and drying. Thebase and the alcohol can be easily regenerated from the filtrate.

[0034] The aqueous solution of hydrogen peroxide generally contains from1 to 50% by weight, preferably 5 to 30% by weight, and most preferably10 to 25% by weight, of hydrogen peroxide.

[0035] The oxidation of the 6,13-dihydroquinacridone salt to thecorresponding quinacridone by hydrogen peroxide is visually followed bythe color change of the reaction mixture. In general, a small excess ofthe hydrogen peroxide is used. The molar ratio of hydrogen peroxide to6,13-dihydroquinacridone is, for example, 1.1 to 5 moles, preferably 1.2to 3.5 moles, of hydrogen peroxide per mole of 6,13-dihydroquinacridone.

[0036] The presence of an oxidation-promoting amount of the catalystduring the oxidation step leads to a higher yield of quinacridone.Additionally, the presence of the catalyst under the oxidationconditions described above, results in a quinacridone product thatcontains only small amounts of quinacridonequinone, for examplecontaining less than 3% by weight of quinacridonequinone. However, minoramounts of quinacridonequinone are tolerated in the product as long asits presence does not substantially reduce the saturation of the finalquinacridone pigment.

[0037] Any compound capable of catalyzing the oxidation of6,13-dihydroquinacridone under the present reaction conditions can beutilized as the catalyst. Particularly suitable catalysts used in theinventive process are, for example, the quinone compounds used for theair oxidation of 6,13-dihydroquinacridone to quinacridone. Such quinonecatalysts are well known in the art. In particular, suitable catalystsinclude anthraquinone compounds, especially anthraquinone, andanthraquinone sulfonic acid derivatives, such asanthraquinone-2,7-disulfonic acid or anthraquinone-2-sulfonic acid, orsalts thereof, in particular the sodium or potassium salts, especiallyanthraquinone-2,7-sulfonic acid, disodium or dipotassium salt. Thequinone catalyst is present in the reaction medium in an amounteffective to catalyze the oxidation reaction, for example from 0.005 to0.15 times the weight of 6,13-dihydroquinacridone, and most preferably0.01 to 0.05 times the weight of 6,13-dihydroquinacridone.

[0038] Without limiting this invention to any particular theory, it isbelieved that the quinone catalyst acts to oxidize the6,13-dihydroquinacridone and is itself reduced to the correspondingleuco compound, which is then regenerated by the hydrogen peroxide.

[0039] Depending on the composition of the liquid phase, therecrystallization time and temperature, as well as the use or nonuse ofparticle growth inhibitors, the chemical structure of such particlegrowth inhibitor, transparent smaller particle size or opaque largerparticle size beta quinacridone crystals in a platelet, needle, cubic,leaflet, prismatic and other geometric forms are generated. Lowertemperatures and the use of particle growth inhibitors favor atransparent product, while higher temperatures and without particlegrowth inhibitors favor a more opaque product.

[0040] Generally, the beta quinacridone crystals generated according tothe inventive process are in a platelet, leaflet shape. The specificsurface area of the beta quinacridones prepared according to the presentprocess are in the range of 2 to 30 m²/g without the use of a particlegrowth inhibitor, and 30 to 65 m²/g when the oxidation reaction iscarried out in the presence of a particle growth inhibitor.

[0041] Additionally, the inventive process selectively oxidizes the6,13-dihydroquinacridone to the corresponding quinacridone easily. Theend product normally contains less than 2% by weight of the unreacted6,13-dihydroquinacridone and less than 3% by weight of the over oxidizedquinacridonequinone. Typically, at least 95% by weight, usually 97.5% byweight and above, of the dihydroquinacridone is converted to thecorresponding quinacridone in its beta crystal phase.

[0042] Although the oxidation is carried out in a heterogeneous reactionmedium, the inventive process provides quinacridone pigments with anarrow particle size distribution. Thus, due to their high purity anddesirable narrow particle size distribution, the obtained quinacridonepigments manifest outstanding pigment properties, such as, for example,a high chroma.

[0043] Depending on the end use, it may be advantageous to add textureimproving agents and/or rheology improving agents, for example beforethe isolation of the pigment, preferably by blending into the aqueouspresscake. Suitable texture improving agents are, in particular, fattyacids of not less than 12 carbon atoms, for example lauric, stearic orbehenic acid or the amides or metal salts thereof, preferably calcium ormagnesium salts, as well as plasticizers, waxes, resin acids such asabietic acid or metal salts thereof, colophonium, alkyl phenols oraliphatic alcohols such as stearyl alcohol or vicinal diols such asdodecanediol-1,2, and also modified colophonium/maleate resins orfumaric acid/colophonium resins or polymeric dispersants. The textureimproving agents are preferably added in amounts of 0.1 to 30% byweight, most preferably of 2 to 15% by weight, based on the finalproduct.

[0044] Suitable rheology improving agents are for example known antiflocculating agents, such as quinacridone derivatives like for examplephthalimido methyl-, imidazolyl methyl- or pyrazolyl methyl-quinacridoneor pigment sulfonic acids, which are added preferably in amounts of 2 to10% by weight, most preferably of 3 to 8% by weight, based on the finalproduct.

[0045] Furthermore it is possible to add other colorants like dyes,organic or inorganic pigments or effect pigments or additives such assurfactants, antifoaming agents, inorganic fillers such as talc or mica,UV-absorber, light stabilizers like the HALS, resins or waxes before,during or after the isolation process. The amount of such additives isgenerally 0 to 40% by weight, preferably 0.1 to 20% by weight, based onthe amount of pigment.

[0046] The present beta quinacridone pigment is suitable as coloringmatter for inorganic or organic substrates. It is highly suitable forcoloring high molecular weight materials, which can be processed tofibers, casted and molded articles or which are used in ink and coatingcompositions such as solvent or water based coatings, conventionallyemployed in the automobile industry, especially in acrylic/melamineresin, alkyd/melamine resin or thermoplastic acrylic resin systems, aswell as in powder coatings and UV/EB cured coating systems.

[0047] Suitable high molecular weight organic materials includethermoplastics, thermoset plastics or elastomers, for example, celluloseethers; cellulose esters such as ethyl cellulose; linear or crosslinkedpolyurethanes; linear, crosslinked or unsaturated polyesters;polycarbonates; polyolefins such as polyethylene, polypropylene,polybutylene or poly-4-methylpent-1-ene; polystyrene; polysulfones;polyamides; polycycloamides; polyimides; polyethers; polyether ketonessuch as polyphenylene oxides; and also poly-p-xylene; polyvinyl halidessuch as polyvinyl chloride, polyvinylidene chloride, polyvinylidenefluoride or polytetrafluoroethylene; acrylic polymers such aspolyacrylates, polymethacrylates or polyacrylonitrile; rubber; siliconepolymers; phenol/formaldehyde resins; melamine/formaldehyde resins;urea/formaldehyde resins; epoxy resins; styrene butadiene rubber;acrylonitrile-butadiene rubber or chloroprene rubber; singly or inmixtures.

[0048] Generally, the inventive beta quinacridone pigment is used in aneffective pigmenting amount, for example, of 0.01 to 30% by weight (upto 70% by weight for masterbatches), preferably 0.1 to 10% by weight,based on the weight of the high molecular weight organic material to bepigmented. Thus, the present invention also relates to a pigmentedplastic composition, which comprises a plastic material, and aneffective pigmenting amount of a beta quinacridone pigment preparedaccording to a process of the present invention, and to a process forpreparing said pigmented plastic compositions.

[0049] The present beta quinacridone pigment is easily dispersible andcan be readily incorporated into organic matrixes to provide homogenouscolorations possessing high saturation, high color strength andexcellent light and weather fastness properties.

[0050] The high molecular weight organic materials are pigmented withthe pigments of the present invention by mixing the pigments, if desiredin the form of a masterbatch, into substrates using high sheartechniques including roll mills or a mixing or grinding apparatus. Thepigmented material is then brought into the desired final form by knownmethods, such as calendering, pressing, extruding, brushing, casting orinjection molding.

[0051] The following examples further describe embodiments of thisinvention. In these examples all parts given are by weight unlessotherwise indicated. The X-ray diffraction patterns are measured on aRIGAKU GEIGERFLEX diffractometer type D/MaxII v BX. The surface areasare measured by the BET method.

EXAMPLES Example 1

[0052] A one-liter flask equipped with a thermometer, stirrer andcondenser is charged with 50 g 6,13-dihydroquinacridone, 180 mlmethanol, 1 g polyvinyl pyrrolidone powder (Luviskol® K-30/BASF) arestirred at 20 to 27° C. for 10 minutes. 73 g 50% aqueous sodiumhydroxide are added. The mixture is stirred under a slow flow ofnitrogen at 50-53° C. for 50 minutes then heated to reflux. 1.2 g2,7-anthraquinone disulfonic acid is added as catalyst. 76 g of anaqueous 18.9% hydrogen peroxide solution is added into the reactionmixture with a peristaltic pump at a pumping rate of 0.4 ml/minute. Theresulting pigment suspension is further stirred for 10 minutes at refluxthen diluted with 100 ml cold water and filtered. The presscake iswashed with hot water then dried, yielding a bronze colored largeparticle size beta quinacridone.

Comparative Example 1

[0053] Example 1 is repeated, however, with the difference that nopolyvinyl pyrrolidone is added. Red gamma quinacridone is obtained.

Example 2

[0054] A one-liter flask equipped with a stirrer, thermometer, condenserand drying tube is charged with 200 ml concentrated (95-98%) sulfuricacid. 31.2 g unsubstituted quinacridone (Cromophtal® Red 2020, CibaSpecialty Chemicals Inc.) is added at a temperature below 45° C. and themixture is stirred for 10 minutes at 40-45° C. to dissolve the pigment.

[0055] 39.7 g of a wet naphthalene sulfonic acid sodium salt presscakewith a solid content of 58%, a mixture containing 80% 1-naphthalenesulfonic acid sodium salt and 20% 2-naphthalene sulfonic acid sodiumsalt (Shanghai Shen Li Chemical Factory) is added at a temperature below45° C. and the mixture is stirred for 15 minutes at 40 to 45° C.followed by the rapid addition of 3.2 g para formaldehyde. The reactionmixture is stirred for one hour at 58-60° C., then poured into 2.5 l ofice water. The violet precipitate is stirred for 1 hour at 5-20° C.,then filtered. The residue is washed with water to a pH of 2.5 and keptas presscake with a solid content of 13% by weight.

[0056] Around 0.5 g of the press cake are reslurried in 20 ml hot wateryielding a bluish red colored liquid. It is filtered to remove littleaggregated material. The filtrate is red and appears a dye solution.However, the electron micrograph shows the quinacridone in nanosizeparticle form with an average particle size of 10 to 25 nm.

Example 3

[0057] A one liter flask equipped with a thermometer, stirrer andcondenser is charged with 50 g 6,13-dihydroquinacridone, 200 mlmethanol, 1 g polyvinyl pyrrolidone powder (Luviskole K-30, BASF) and 7g of the aqueous presscake of the nanosize quinacridone naphthalenesulfonic acid formaldehyde polymer mixture according to Example 2 arestirred at 20-27° C. for 10 minutes. 50 g 50% aqueous sodium hydroxideare added. The mixture is stirred under a slow flow of nitrogen at50-53° C. for one hour. 0.8 g anthraquinone mono sulfonic acid sodiumsalt is added as catalyst and the reaction mixture is heated to refluxfor 10 minutes. 76 g of an aqueous 18.9% hydrogen peroxide solution areadded into the reaction mixture with a peristaltic pump at a pumpingrate of 0.4 ml/minute. Then after 25 minutes addition time 1.6 gphthalimidomethyl-quinacridone are introduced into the reaction mixturefollowed by continuing the hydrogen peroxide addition while maintainingreflux and a slow nitrogen flow. The resulting violet suspension isfurther stirred for 10 minutes at reflux then diluted with 100 ml coldwater and filtered. The presscake is washed with hot water then dried,yielding a small particle size violet beta quinacridone.

[0058] The product shows a high purity and less than 0.1% remaining6,13-dihydro-quinacridone as determined spectrophotometrically. TheX-ray diffraction pattern of the pigment shows the characteristics of abeta quinacridone. When incorporated into automotive paints, the productleads to a strong violet color dispersion with excellent Theologicalproperties and an attractive color appearance when drawn on a contrastcarton, which dispersion can easily be sprayed on metallic panelsgenerating coatings of excellent durability (a topcoat may then furtherbe applied in conventional manner, for example by the wet-on-wettechnique).

Example 4

[0059] A one liter flask equipped with a thermometer, stirrer andcondenser is charged with 50 G 6,13-dihydroquinacridone, 200 mlmethanol, 1 g polyvinyl pyrrolidone powder (Luviskol® K-30, BASF) and 7g of the aqueous presscake of the nanosize quinacridone naphthalenesulfonic acid formaldehyde polymer mixture according to Example 2, arestirred at 20-27° C. for 10 minutes. 73 g 50% aqueous sodium hydroxideare added. The mixture is stirred under a slow flow of nitrogen at50-53° C. for 30 minutes. 4.6 g of a 37% aqueous solution ofanthraquinone-2,7-disulfonic acid disodium salt is added as catalystfollowed by 10 ml water and the reaction mixture is heated to reflux for40 minutes. 79 g of an aqueous 18.2% hydrogen peroxide solution areadded into the reaction mixture with a peristaltic pump at a pumpingrate of 0.4 ml/minute. Then after 20 minutes addition time 2.3 gphthalimido methyl-quinacridone and after another 70 minutes 0.3 gphthalimido methyl-quinacridone are introduced into the reaction mixturefollowed by continuing the hydrogen peroxide addition while maintainingreflux and a slow nitrogen flow. The resulting violet suspension isfurther stirred for 10 minutes at reflux then diluted with 100 ml coldwater and filtered. The presscake is washed with hot water then dried,yielding a violet quinacridone.

[0060] The product shows a high purity and less then 0.1% remaining6,13-dihydro-quinacridone as determined spectrophotometrically. Thex-ray diffraction pattern of the pigment shows the characteristics of abeta quinacridone. The specific surface area as measured by the BETmethod is 52 m²/g. Incorporated in automotive paints or plastics theproduct gives a strong violet color with excellent pigment properties.

Example 5

[0061] A small particle size beta quinacridone with an average particlesize of 0.05 to 0.1 μm as described in U.S. Pat. No. 2,844,485 isprepared according to U.S. Pat. No. 3,607,336 by a high turbulencedrowning process from concentrated sulfuric acid precipitation startingfrom a beta quinacridone crude with an average particle size of 2 to 8μm, yielding a violet particulate quinacridone having an averageparticle size in the range of 0.005 to 0.2 μm, which is kept aspresscake.

Example 6

[0062] A one liter flask equipped with a thermometer, stirrer andcondenser is charged with 50 g 6,13-dihydroquinacridone, 200 mlmethanol, 1 g polyvinyl pyrrolidone powder (Luviskole K-30, BASF) and 4g of the aqueous presscake with a solid content of 20% by weight of theparticulate quinacridone according to Example 5 are stirred at 20-27° C.for 10 minutes. 77 g 50% aqueous sodium hydroxide are added. The mixtureis stirred under a slow flow of nitrogen at 50-53° C. for 30 minutes.4.6 g of a 37% aqueous solution of anthraquinone-2,7-disulfonic aciddisodium salt is added as catalyst followed by 10 ml water and thereaction mixture is heated to reflux for 30 minutes. 79 g of an aqueous18.2% hydrogen peroxide solution are added into the reaction mixturewith a peristaltic pump at a pumping rate of 0.4 ml/minute, wherebyafter 5 minutes addition time 3.2 g phthalimido methyl-quinacridone areintroduced into the reaction mixture followed by continuing the hydrogenperoxide addition while maintaining reflux and a slow nitrogen flow. Theresulting violet suspension is further stirred for 10 minutes at refluxthen diluted with 100 ml cold water and filtered. The presscake iswashed with hot water then dried, yielding a violet quinacridone with anaverage particle size of 0.05 to 0.25 μm.

[0063] The product shows a high purity and less than 0.1% remaining6,13-dihydro-quinacridone as determined spectrophotometrically. FIG. 1shows the X-ray diffraction pattern of the pigment with thecharacteristic pattern of a beta quinacridone: Scattering Angle [°2θ]Relative Intensity [%] 6.0 100 11.9 39 16.1 26 17.8 9 22.1 21 23.8 1527.3 71 29.7 9

[0064] Incorporated in automotive paints or plastics, the productproduces a strong violet color with excellent pigment properties.

Example 7

[0065] 63.0 g of polyvinyl chloride, 3.0 g epoxidized soy bean oil, 2.0g of barium/cadmium heat stabilizer, 32.0 g dioctyl phthalate and 1.0 gof the violet beta quinacridone pigment according to Example 6 are mixedtogether in a glass beaker using a stirring rod. The mixture is formedinto a soft PVC sheet with a thickness of about 0.4 mm by rolling for 8minutes on a two roll laboratory mill at a temperature of 160° C., aroller speed of 25 rpm and friction of 1: 1.2, by constant folding,removal and feeding. The resulting soft PVC sheet is colored in anattractive violet shade and has excellent fastness to heat, light andmigration.

Example 8

[0066] 5 g of the violet beta quinacridone pigment according to Example4, 2.65 g Chimassorb® 944LD (hindered amine light stabilizer), 1.0 gTinuvin® 328 (benzotriazole UV absorber) and 2.0 g Irganox® B-215 Blend(antioxidant, all from Ciba Specialty Chemicals Inc.), are mixedtogether with 1000 g of high density polyethylene at a speed of 175-200rpm for 30 seconds after flux. The fluxed, pigmented resin is chopped upwhile warm and malleable, and then fed through a granulator. Theresulting granules are molded on an injection molder with a 5 minutedwell time and a 30 second cycle time at temperatures of 200, 250 and300° C. Homogeneously colored chips, which show a violet color with anexcellent light stability and practically no color differences betweenthe different temperatures, are obtained.

Example 9

[0067] This Example illustrates the incorporation of the inventive betaquinacridone according to Example 3 into an automotive paint system.

[0068] Millbase Formulation

[0069] A pint jar is charged with 30.9 g acrylic resin, 16.4 g ABdispersant consisting of 45% of an acrylic resin in toluene, and 42.8 gsolvent (Solvesso™ 100 from American Chemical). 30.0 g beta quinacridoneaccording to Example 6 and 980 g of 4 mm diameter steel diagonal rodsare added. The jar is shaken on a Skandex™ shaker for 5 hours. Themillbase contains 25.0% pigment with a pigment/binder ratio of 0.5.

[0070] Masstone Color

[0071] 48.9 g of the above millbase, 71.7 g of a clear 47.8% solidsunpigmented resin solvent solution containing a melamine resin catalyst,a non-aqueous dispersion resin and a UV absorber, and 29.4 g of a clearunpigmented 58% solids unpigmented polyester urethane resin solventsolution, are mixed and diluted with a solvent mixture comprising 76parts xylene, 21 parts butanol and 3 parts methanol to a spray viscosityof 20-22 seconds as measured by a #2 Fisher Cup. The resin/pigmentdispersion is sprayed onto a panel twice at 1½ minute intervals asbasecoat. After 2 minutes, the clearcoat resin is sprayed twice at 1½minute intervals onto the basecoat. The sprayed panel is then flashedwith air in a flash cabinet for 10 minutes and then “baked” in an ovenat 129° C. for 30 minutes, yielding a violet colored panel. The coatedpanel has excellent weatherability.

Example 10

[0072] 1000 g of polypropylene granules (Daplen PT-55®, Chemie Linz) and10 g of the beta quinacridone pigment according to Example 4 arethoroughly mixed in a mixing drum. The granules so obtained aremelt-spun at 260-285° C. to violet filaments of good light fastness andtextile fiber properties.

I claim:
 1. A process for preparing a beta quinacridone of formula (I)

by the oxidation of a salt of a corresponding 6,13-dihydroquinacridoneof formula (III)

which comprises oxidizing the 6,13-dihydroquinacridone salt withhydrogen peroxide in the presence of a catalyst and 0.2 to 4% by weight,based on the 6,13-dihydroquinacridone of a polyvinyl pyrrolidone.
 2. Aprocess according to claim 1, wherein 0.5 to 3% by weight based on the6,13-dihydroquinacridone of a polyvinyl pyrrolidone is present.
 3. Aprocess of claim 1, wherein the 6,13-dihydroquinacridone salt is analkali metal salt.
 4. A process of claim 3, wherein the6,13-dihydroquinacridone salt is a disodium or dipotassium salt.
 5. Aprocess of claim 1, wherein particulate quinacridone having an averageparticle size of below 0.2 micron and which is in the beta crystal phaseis present during oxidation.
 6. A process of claim 5, wherein a mixtureof the particulate quinacridone as a nanosize quinacridone and anaphthalene sulfonic acid formaldehyde polymer is present duringoxidation.
 7. A process of claim 5, wherein the particulate quinacridoneis prepared from sulfuric acid precipitation.
 8. A process of claim 5,wherein the particulate quinacridone is present at a concentration from0.1 to 10 percent based on 6,13-dihydroquinacridone.
 9. A process ofclaim 1, wherein the oxidation step is carried out by combining a slurryconsisting essentially of the 6,13-dihydroquinacridone salt, thecatalyst, the polyvinyl pyrrolidone, a base and a liquid phase, with anaqueous solution of hydrogen peroxide.
 10. A process of claim 9, whereinthe liquid phase consists essentially of from about 20 to 750 parts byweight of water and from about 50 to 750 parts by weight of a loweralcohol per 100 parts by weight of 6,13-dihydroquinacridone.
 11. Aprocess of claim 10, wherein the liquid phase consists essentially offrom 40 to 600 parts by weight of water and from 100 to 600 parts byweight of the alcohol per 100 parts by weight of6,13-dihydroquinacridone.
 12. A process of claim 10, wherein the alcoholis a C₁ to C₃ alcohol.
 13. A process of claim 12, wherein the alcohol ismethanol.
 14. A process of claim 9, wherein wherein the base is analkali metal hydroxide, which is present in an amount of from 1 to 8moles per mole of the 6,13-dihydroquinacridone.
 15. A process of claim14, wherein the alkali metal hydroxide is present in an amount of from2.2 to 7 moles per mole of the 6,13-dihydroquinacridone.
 16. A processof claim 14, wherein the alkali metal hydroxide is sodium or potassiumhydroxide, or a mixture thereof.
 17. A process of claim 9, wherein thecatalyst is a quinone compound.
 18. A process of claim 17, wherein thecatalyst is selected from the group consisting of anthraquinone,anthraquinone monosulfonic acid and anthraquinone disulfonic acid, or asalt thereof.
 19. A process of claim 18, wherein the catalyst isanthraquinone-2-sulfonic acid or anthraquinone-2,7-disulfonic acid orits sodium or potassium salts.
 20. A process of claim 9, wherein thecatalyst is present in an amount of from 0.005 to 0.15 times the weightof the 6,13-dihydroquinacridone.
 21. A process of claim 9, wherein theoxidation step is carried out by combining a 1 to 50 percent by weightaqueous solution of hydrogen peroxide with a slurry consistingessentially of the 6,13-dihydroquinacridone, the catalyst, the base andthe liquid phase.
 22. A process of claim 21, wherein the aqueoushydrogen peroxide solution has a concentration of 5 to 30 weight percentof hydrogen peroxide.
 23. A process of claim 9, wherein 1.1 to 5 molesof the hydrogen peroxide are combined per mole of6,13-dihydroquinacridone.
 24. A process of claim 9, wherein the aqueoussolution of hydrogen peroxide is added to the slurry over a timeinterval of from 5 minutes to 6 hours at elevated temperature and thereaction medium is subsequently maintained, with stirring, at anelevated temperature for from 5 minutes to 4 hours to complete theoxidation and promote pigment recrystallization.
 25. A process of claim9, wherein the reaction medium is maintained at a temperature of from50° C. to reflux temperature for from 5 minutes to 2 hours.
 26. Aprocess of claim 9, wherein the oxidation step is carried out in thepresence of from 0.05 to 10% by weight based on the6,13-dihydroquinacridone of a particle growth inhibitor.
 27. A processof claim 26, wherein said particle growth inhibitor isphthalimidomethyl-, imidazolyl methyl-, pyrazolyl methyl-quinacridone,quinacridone mono sulfonic acid and its salts.